Body Mass Index

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Body Mass Index

Body weight is used as an indicator of an individual's health. It is usually compared to tables that list "ideal" or "desirable" weight ranges for specific heights. Some of these tables use values gathered from research studies, while some include the heights and weights of individuals who have bought life insurance (e.g., the Metropolitan Height and Weight Tables). An individual's weight can be described as a percentage of the ideal or desirable weight listed, and can also be categorized as healthy, underweight, over-weight , or obese . An additional method of comparing an individual to a population group is with the body mass index .

Body mass index (BMI) is an estimate of body composition that correlates an individual's weight and height to lean body mass. The BMI is thus an index of weight adjusted for stature. Body mass index is figured by dividing weight in kilograms by height in meters squared and multiplying by 100. It can also be figured by dividing weight in pounds by height in inches squared and multiplying by 705. High values can indicate excessive fat stores, while low values can indicate reduced fat stores. In this way, the BMI
is a diagnostic tool for both obesity and protein-energy malnutrition . The BMI has also been associated with mortality, with lower values generally correlating with longer life.

However, when evaluating the BMI, several characteristics of an individual need to be known. An individual's age, gender, ethnicity, and level of fitness must be considered when using BMI to determine health risk. Also, the significance of the BMI is affected by disease state and hydration status. As with most assessment tools, the BMI is most effective when used in conjunction with other measurements.

Tables are available to identify the significance of the BMI. Calculations based on values for ideal body weight suggest the BMI for normal men and women should be in the range of 19 to 27 kg/m2. This range corresponds to the 25th to 75th percentile values recorded from adults followed in the 1971–1974 National Health and Nutrition Examination Survey (NHANES). Tables also list levels of protein-energy malnutrition and obesity. These values were determined by research in which height, weight, and age were associated with functional measurements and health outcomes.

A BMI between 13 and 15 corresponds to 48 to 55 percent of desirable body weight for a given height and describes the lowest body weight that can sustain life. Body weight at this level consists of less than 5 percent fat. The maximum survival body weight is about 500 kg, which corresponds to a BMI of about 150.

Research with children indicates annual increases in BMI are usually due to increases in lean mass rather than fat tissue. Not until late adolescence does fat mass begin to affect the BMI—and adult values begin to be achieved.

There is a strong correlation between BMI and total fat mass, though individual variation in body type or height can cause misclassification. Unfortunately,

Body mass index equation

Significance of BMI values for adults

Condition Indicated

Men

Women

Protein-calorie malnutrition

< 17

< 17

Underweight

< 20

< 19

Acceptable weight

20.7 – 27.8

19.1 – 27.3

Intervention indicated

> 26.4

> 25.8

Obese

> 27.8

> 27.3

Severely obese

> 31.1

> 32.2

Morbidly obese

> 45.4

> 44

Normal BMI Values for Infants and Children

Infants (at birth)

13

1 year

18

6 years

15

the same BMI value can correlate with a range of body-fat percentage. For example, athletes usually have large skeletal muscles (which weigh more than fat) and therefore a high BMI, but they are not obese. Shorter individuals can also be identified as obese, since their BMIs are usually high. An older individual may have a higher body-fat percentage than a younger individual, but have the same BMI. Adult females can have a BMI of 20, which correlates to a body-fat percentage of 13 to 32 percent, while adult males can have a BMI of 27 and a body-fat percentage of 10 to 31 percent.

Findings from the third NHANES (1988–1994) describe misidentification of the elderly when self-reported height, rather than measured height, is used in the BMI equation. Height decreases over an individual's lifetime due to vertebral compression, loss of muscle tone, and postural slump. An individual may, therefore, report a height that is no longer accurate, and the resulting value will be lower than the value that actually describes the individual, possibly leading to the wrong intervention.

Research has shown that both high BMIs and low BMIs can indicate increased morbidity and mortality. A low BMI, usually an indication of protein-energy malnutrition or the effects of wasting or a disease process, is a significant predictor of mortality among young and old hospitalized patients. A high BMI has been shown to be predictive of mortality only among young hospitalized patients, usually an effect of cardiovascular disease and obesity. Risk of mortality is only slightly elevated at the highest BMI for elderly hospitalized patients.

Because ethnicity has been shown to require adjustments to the levels of concern for the BMI, care must be taken when comparing different population groups. For example, Asian populations may require a lower BMI to describe health risk, while Pacific populations, specifically Hawaiian, may require a higher threshold to indicate that an individual is at risk. This variation can be explained by body type.

BMI and waist circumference have been used to evaluate health risks associated with overweight and obesity. Because both are easy measures to
do, standardization of both are encouraged for widespread use as a reference. Additionally, the two measurements have been used in an algorithm with a cardiovascular risk index to determine which individuals would benefit most from weight loss.

BMI is an easy measurement to make—only requiring a tape measure, scale, and, perhaps, a calculator. However, for individuals who have trouble standing up straight for an accurate height measurement—either from disease process, weakness, or kyphosis (abnormal backward curvature of the spine)—BMI may not be an easy or accurate assessment tool to use. Comparisons between BMI and mid-upper arm circumference (MUAC) measurements show that they identify the same level of malnutrition in individuals. MUAC is also easily measured (it requires only a tape measure), and it is a good indicator of change in body weight and muscle mass. Standardization of these two assessment tools for reference would benefit the science of nutrition assessment.

Kuczmarski, Marie Fanelli (2001). "Effects of Age on Validity of Self-Reported Height, Weight, and Body Mass Index: Findings from the Third National Health and Nutrition Survey, 1988–1994." Journal of the American Dietetic Association 101(1):28–34.

Body Mass Index

Body Mass Index

Body Mass Index, or BMI, is a common measure of weight status in adults. BMI can be calculated by multiplying weight in pounds by 703, divided by height in inches squared, and it serves as an index of weight-for-height measured in kg/m2. BMI indicates overweight between 30 kg/m2 and 34.9 kg/m2; obesity between 35 kg/m2 and 39.9 kg/m2; and clinically severe obesity above 40 kg/m2. BMI is an indirect estimate of body fat and is highly correlated with body fat at about .7 (Gray and Fujioka 1991). Although there are more accurate measures of body fat (e.g., underwater weighing and DXA), they are more expensive, inaccessible, and cumbersome compared to BMI (Blew et al. 2002). The widespread use of BMI is likely due to its cost-effectiveness and ease of calculation.

The measurement and definition of overweight and obesity has varied over time. For much of the twentieth century, physicians and researchers referenced Metropolitan Life Insurance Company (MLIC) tables, which recommended ideal weight-for-height. The MLIC tables suffered from limitations (e.g., unstandardized and inaccurate measurement protocols) that prompted the government to adjust the weight guidelines in the 1980s (Kuczmarski and Flegal 2000). In the mid-1980s BMI became the preferred measurement of weight status, and recommendations were based upon data from national epidemiological surveys such as the National Health and Nutrition Examination Survey. BMI emerged in the first annual federal report on the prevalence of obesity in the United States, and a National Institutes of Health (NIH) panel defined overweight in terms of sex-specific BMI cutoffs (Kuczmarski and Flegal 2000; National Center for Health Statistics 1984; National Institutes of Health Consensus Development Panel 1985).

The current classification system adopted by the National Heart, Lung, and Blood Institute uses BMI to determine weight category. Classification of weight status is important because numerous medical comorbidities are associated with increased BMI. The BMI cutoff for overweight has decreased over time from 30 to 27, and most recently 25. Further, BMI provides a relative index of growth stunting, a condition that may result in significant developmental delays and adverse physiological effects (Dickerson 2003).

Weight category

BMI (kg/m2)

Underweight

<18.5

Normal

18.5-24.9

Overweight

25.0-29.9

Obesity Class I

30.0-34.9

Obesity Class II

35.0-39.9

Obesity Class III

40+

There is empirical evidence that BMI may be more predictive of body fatness in certain subgroups (e.g., younger adults, Caucasians) than others (Baumgartner, Heymsfield, and Roche 1995; Gallagher et al. 1996). Thus, two individuals with an identical BMI may have a different percentage of body fat depending on factors such as age, gender, body shape, and ethnicity (Prentice and Jebb 2001). BMI also overestimates body fat in persons who are very muscular (e.g., athletes), does not distinguish lean mass (muscle and bone) from fat mass, and does not determine the distribution of body fat. In children, BMI must be adjusted for growth. Despite these shortcomings, BMI classifications are still valuable for research and health care.

BMI is used to diagnose and make treatment recommendations. Epidemiological studies measure BMI to identify population trends in growth retardation and obesity along with associated adverse health consequences. Mounting evidence indicates an increased risk of mortality among obese individuals. Increased BMI has been associated with medical comorbidities including cardiovascular disease, reduced fertility, sleep apnea, metabolic syndrome, hypertension, type 2 diabetes, and certain cancers. In addition to medical risks, evidence suggests that there is a powerful social stigma associated with obesity. Discrimination affects overweight individuals in numerous facets of life, including employment, education, and psychological well-being (Friedman et al. 2005; Puhl and Brownell 2003).

Kuczmarski, Robert J., Katherine M. Flegal. 2000. Criteria for Definition of Overweight in Transition: Background and Recommendations for the United States. American Journal of Clinical Nutrition 72: 1074–1081.

National Center for Health Statistics. 1984. Health, United States, 1984. Washington, DC: U.S. Government Printing Office.

National Center for Health Statistics Consensus Development Panel on the Health Implications of Obesity. 1985. Health Implications of Obesity. Annals of Internal Medicine 103: 1073–1077.

National Heart, Lung, and Blood Institute. 1998. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. Rockville, MD: National Institutes of Health.

Seidell, Jaap C., Henry S. Kahn, David F. Williamson, et al. 2001. Report from a Centers for Disease Control and Prevention Workshop on Use of Adult Anthropometry for Public Health and Primary Health Care. American Journal of Clinical Nutrition 73: 123–126.

U.S. Department of Agriculture and U.S. Department of Health and Human Services. 1980. Nutrition and Your Health: Dietary Guidelines for Americans. Washington, DC: U.S. Government Printing Office.

Kelli Friedman

Erin E. Martinez

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body mass index

body mass index (BMI) n. the weight of a person (in kilograms) divided by the square of the height of that person (in metres): used as an indicator of whether or not a person is over- or underweight. A BMI of between 20 and 25 is considered normal, between 25 and 30 is overweight, and greater than 30 indicates clinical obesity.

Calculation of body mass index

BMI can be calculated using metric units (kilograms and metres) or imperial units (pounds and inches). The metric system is more widely used.

Metric calculation

Imperial calculation

Weight in pounds is divided by height in inches squared and multiplied by 703, i.e. [weight (kg)/height2 (in)]×703

For example:

height=144 lb, height=60kg

Calculation:

[144ɇ(60)2]×703=28.12

Interpretation of body mass index

The range within which a person's BMI falls will help determine whether they are of a healthy weight for their height. The ranges are as follows:

Weight status

BMI

Starvation

<15

Underweight

<18.5

Normal weight

18.5–24.9

Overweight

25–29.9

Obesity

30–40

Morbid obesity

>40

The first of the example BMIs given above, 24.6, would fall within the ‘normal’ range, indicating good health with no need to lose weight.

The second example BMI, 28.12, would fall within the ‘overweight’ range. A person with a BMI of 25 or over is probably overweight, with a greater risk of developing heart disease, osteoarthritis, diabetes, high blood pressure, stroke, some cancers, and other diseases. Steps should be taken to lose weight, which can include a healthier diet and regular exercise. A BMI of 18.5 or under may indicate that the person is deliberately restricting their food intake in order to achieve a desired degree of thinness. This is unhealthy and may lead to such health problems as malnutrition and osteoporosis, which are associated with the eating disorders anorexia nervosa and bulimia nervosa. A BMI of 17.5 or less is one of the criteria stipulated by the World Health Organization for a diagnosis of anorexia nervosa. For someone with a BMI of 30 or over, advice from a health professional about how to lose weight may be beneficial. Caution must be exercised when using the BMI as it is a screening tool, not a diagnostic tool.

BMI and children

Weight status

Centile range

Underweight

Less than the 5th centile

Healthy weight

5th percentile to less than the 85th centile

At risk of overweight

85th to less than the 95th centile

Overweight

Equal to or greater than the 95th centile

Although the BMI number is calculated in the same way for children and adults, the criteria used to interpret the meaning of the BMI number for children and teenagers are different from those used for adults. For children and teens, BMI age- and sexspecific centiles are used for the following reasons:• the amount of body fat changes with age• the amount of body fat differs between girls and boys• healthy weight ranges change with each month of age for each sex• healthy weight ranges change as height increases

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body mass index

body mass index (BMI) An index of fatness and obesity. The weight (in kg) divided by the square of height (in m). The acceptable (desirable) range is 20–25. Above 25 is overweight, and above 30 is obesity. BMI below the lower end of the acceptable range indicates undernutrition and wasting. Also called Quetelet's index. See also weight, ideal.

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